This invention relates to a heat pump that is operable in both a cooling and a heating mode, and wherein a discharge valve is controlled to increase and modulate the heating capacity of the heat pump.
Refrigerant systems are utilized to control the temperature and humidity of air in various indoor environments to be conditioned. In a typical refrigerant system operating in the cooling mode, a refrigerant is compressed in a compressor and delivered to a condenser (or an outdoor heat exchanger in this case). In the condenser, heat is exchanged between outside ambient air and the refrigerant. From the condenser, the refrigerant passes to an expansion device, at which the refrigerant is expanded to a lower pressure and temperature, and then to an evaporator (or an indoor heat exchanger). In the evaporator heat is exchanged between the refrigerant and the indoor air, to condition the indoor air. When the refrigerant system is operating, the evaporator cools the air that is being supplied to the indoor environment.
The above description is of a refrigerant system being utilized in the cooling mode of operation. In the heating mode, the refrigerant flow through the system is essentially reversed. The indoor heat exchanger becomes the condenser and releases heat into the environment to be conditioned (heated in this case) and the outdoor heat exchanger serves the purpose of the evaporator and exchangers heat with a relatively cold outdoor air. Heat pumps are known as the systems that can reverse the refrigerant flow through the refrigerant cycle, in order to operate in both heating and cooling modes. This is usually achieved by incorporating a four-way reversing valve (or an equivalent device) into the system design, with the valve located downstream of the compressor discharge port. The four-way reversing valve selectively directs the refrigerant flow through the indoor or outdoor heat exchanger when the system is in the heating or cooling mode of operation, respectively. Furthermore, if the expansion device cannot handle the reversed flow, than a pair of expansion devices, each along with a check valve, can be employed instead.
Heat pumps are intended to replace a furnace, such that a single unit can provide the function of both the air conditioner and the furnace. However, heat pumps have not been widely adopted in colder climates. The major reasons for this slow adoption is the concern that the heat pump cannot provide adequate heat in colder climates and/or the temperature of the heated air delivered to the conditioned environment is too cold (so called “cold blow”) and uncomfortable to the end user. An additional drawback is that to compensate for the lack of heating capacity, the system needs to rely on separate heaters. Since a heater delivers a predetermined amount of heating capacity, the system must be cycled OFF when the desired indoor temperature is reached and cycled back ON when the temperature falls below the desired value. The unit cycling is inefficient, prone to reliability problems, magnifies temperature variations in the conditioned space and causes discomfort to the end user.